Acetabular shell removal tool

ABSTRACT

A tool for separating an acetabular shell of a hip prosthesis from the surrounding pelvic bone includes a fixture which attaches to the acetabular shell and has a chisel guide mounted on it. A chisel associated with the chisel guide is curved to conform to the outer periphery of the acetabular shell, and the chisel guide causes the chisel to circumscribe the acetabular shell as the chisel is inserted between the acetabular shell and the pelvis.

BACKGROUND OF THE INVENTION

Total hip anthroplasty has become a relatively common procedure. It alsois becoming more common to replace the prosthesis after a period oftime. A difficulty in doing this is that the acetabular shell of theprosthesis will become completely embedded in the surrounding bone andneeds to be cut free. The traditional method of accomplishing this hasbeen to use hand-held, curved chisels and a mallet to cut and pry theacetabular shell away from the bone. A problem with this process is thatit is difficult to keep the chisel precisely at the periphery of theacetabular shell and often excess bone is removed with the shell.

The X-PLANT acetabular shell removal tool sold by the Zimmer Companyreduces this problem to some degree. The X-PLANT removal tool provides acurved chisel having the same contour as the outer periphery of theacetabular shell. The chisel is attached to a shaft having a ball at itsextremity which is sized to be inserted into the socket in thepolyethylene liner which is located in the acetabular shell to receivethe femural head of the prosthesis. As the tool is manipulated andstruck with a mallet to break the acetabular shell free from thesurrounding bone, the ball pivots in the socket and keeps the chiselblade close to the periphery of the acetabular shell.

While the X-PLANT tool is an improvement over the traditional hand-heldchisels, it does have one significant shortcoming. Often the acetabularshell is attached to the pelvis with screws which extend through theshell and into the surrounding bone. These screws need to be removedbefore the shell can be removed and doing this requires that thepolyethylene liner be removed from the shell, which often damages themechanism which holds the liner in place. As a result the liner moveswhen the X-PLANT tool is used which prevents the tool from cutting asclose to the periphery of the shell as it otherwise would.

The subject invention overcomes the foregoing shortcomings of the priorart tools for removing the acetabular shell by providing a fixture whichattaches to the shell. The fixture has a chisel guide mounted on it. Thechisel guide causes a chisel to move along a path such that itsubstantially circumscribes the outer surface of the shell when thechisel is moved.

A BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevation view showing a hip prosthesis.

FIGS. 2-7 are side elevation views, at various stages of completeness,showing an acetabular shell removal tool embodying the subjectinvention.

FIG. 8 is a bottom view of a locking plate which is an element of thetool.

FIG. 9 is an exploded view showing the various parts of the tool.

FIGS. 10-12 are side elevation views of another embodiment of the toolin various stages of completeness.

FIG. 13 is a side elevation view of another embodiment of the tool.

FIG. 14 is a perspective view of yet another embodiment of the tool.

FIG. 15 is a side elevation view of the tool of FIG. 14 installed on anacetabular shell.

The foregoing and other objectives, features, and advantages of theinvention will be more readily understood upon consideration of thefollowing detailed description of the invention taken in conjunctionwith the accompanying drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1 of the drawings, a total hip anthroplasty includesplacing an acetabular shell 10 in a mating recess 12 in the pelvis 14.The shell 10 is typically stainless steel, has a semi-spheriodal outersurface 16 which contacts the pelvis and a semi-spheriodal inner surface18 which is spaced inwardly from the outer surface. Located in the shellis a polyethylene liner 20 having a cavity which carries the femuralhead 22 at the extremity of the femural component 23 of the prosthesis.The liner is locked in place in the shell by a locking mechanism, notshown. With time the pelvis bone becomes firmly joined to the shell 10.As a result, when it becomes necessary to remove the shell, the bonemust be broken or cut away from the shell.

The tool of the subject invention facilitates the removal of the shell.Referring to FIGS. 2-9 of the drawings, a first embodiment of the tool24 is utilized where the shell has an opening 26 at its center which wasused to instill the shell. The tool 24 includes an anchor 28, FIG. 2,which attaches to the shell. Located at the inner end of the anchor 28is a seat 30. The seat 30 can either have self-tapping threads orthreads that mate with the existing threads in the opening 26. A hexhead 32 is located at the outer end of the anchor so that a wrench 34can be attached to the anchor to screw the seat 30 into the opening 26.

Referring now also to FIGS. 3 and 4, once the anchor is in place acentering plate 36 is installed on it. The centering plate 36 iscircular in cross-section and has a diameter which causes it to contactthe inner surface 18 of the shell slightly below its outer edge 38.Located in the center of the centering plate 36 is a hole 40. The hole40 and the center portion 42 of the anchor have mating threads. Thus, asthe centering plate is screwed onto the anchor it causes the anchor tobecome centered in the shell and be perpendicular to the outer edge 38of the shell. The centering plate 36 has a series of holes 44 located init and a wrench 46, having pins 48 which engage the holes, fits over theanchor to facilitate screwing the centering plate tightly into theshell. A hex head 49 is placed at the top of the wrench 44 for thispurpose.

Referring now to FIG. 5, a cylindrical locking plate 50 is placed on theanchor outwardly of the centering plate 36. The locking plate has acentral hole 52 having an inner diameter which is larger than the outerdiameter of the anchor 28. The locking plate is large enough that theperiphery of its base 54 rests on the edge 38 of the shell 10. Theportion of the anchor which fits through the hole 52 in the centeringplate has a series of saw-tooth-shaped rings 56 formed in its outersurface. Located in the locking plate 50 are three spring-loaded detents58 having chisel-shaped teeth 60 which engage the saw-tooth rings 56 inthe anchor, FIG. 8. The detents center the locking plate on the anchorand hold it tightly in place against the outer edge 38.

Located in the locking plate 50 is a T-shaped slot 62 which receives achisel guide 64, FIG. 6. Located in the chisel guide is a track 65,which in the embodiment illustrated in FIGS. 2-9 has the same curvatureas the outer surface 16 of the shell. A chisel 66 has a rail 68 attachedto it which fits slidably in the track 65 in the chisel guide. Thechisel has a curved blade 70 which also has the same curvature as theouter surface 16 of the shell. The blade 70 is wider at its base than itis at its tip as can be seen in FIG. 8. The chisel guide and chisel areconfigured such that when the chisel is moved in the chisel guide towardthe pelvis it closely circumscribes the outer surface 16 of the shell,FIG. 7. As will be described more fully below, the chisel does notnecessarily move in the chisel guide. The chisel guide and chisel canmove together as a unit.

Attached to the base of the chisel is a rod 72 having an impact pad 74located at its extremity. The rod is attached to the chisel through ahinge 76. The impact pad 74 can be struck with a mallet to drive thechisel 66 inwardly between the shell 10 and pelvic bone 14 when the rodis in the position shown in FIG. 7. Alternatively, the rod can berotated 180 degrees so that when the impact pad 74 is struck with amallet the chisel is driven in the opposite direction. As will be morefully explained below, the impact pad can be connected to the chiselthrough a non-rotating element also. The entire tool 24 is shownexploded in FIG. 9.

An alternative apparatus for attaching the anchor to the shell is shownin FIG. 13. In this embodiment the anchor 102 has a spring-loadedexpandable mandrel 104 which fits in the opening 26 in the center of theshell 10. A locking mechanism, not shown, locks the mandrel in itsexpanded position once it is installed.

Referring now to FIGS. 10-12, when the acetabular shell 10 does not havean opening in it, a second embodiment of the tool 78 is used. In thisembodiment the tool is attached to the shell 10 through an annular slot80 which is located in the upper portion of the inner surface 18 of theshell to hold the polyethylene liner 20 into the shell. The tool 78includes a clamping ring 82 having an outer diameter which is slightlyless than the diameter of the inner surface of the shell at the locationof the slot 80. The clamping ring 82 has three spring-loaded detents 84having teeth 85 that project from the periphery of the clamping ring andsnap into the slot 80 when the clamping ring is pushed into the shell.The detents 84 are similar to the detents 58. The clamping ring has aplaner top surface 88 which is located below the upper edge 38 of theshell when the clamping ring 82 is inserted into the shell and the teeth85 are seated in the slot 80. Located in the center of the clamping ring82 is a threaded hole 86.

Attached to the clamping ring 82 is a plate 90 which has a T-shaped slot92 that is similar to the slot 62 and receives the chisel guide 66. Anopening 95 having an inwardly conical-shaped upper portion 94 extendsthrough the plate 90. A bolt 96 has a threaded cylindrical lower portion98 that mates with the threaded opening 86 in the plate 90. The upperportion 100 of the bolt 96 is conical and mates with the conical portion94 of the opening 95. Thus, when the bolt 96 is inserted through theopening 95 in the plate 90 and into the threaded hole 86 and tightened,the plate is centered on the clamp ring and pulled tight against theupper edge 38 of the shell, FIG. 11. The chisel 66 is then inserted inthe chisel guide 64 and is operated in the same manner that it is withthe first embodiment of the tool, FIG. 12.

A third embodiment of the invention is shown in FIGS. 14 and 15. In thisembodiment the anchor 106 is similar to the anchor 28 in the firstembodiment except that the portion having the saw-tooth rings isreplaced by a cylindrical pin 108 which extends slightly outwardly fromthe shell 10. A cylindrical mounting plate 110 rests on the upper edge38 of the shell 10 and has a central hole 112 which receives the pin108. Thus, the mounting plate is freely rotatable about the pin 108. Anelongate chisel guide 114, which is mounted on the mounting plate 110,rotates between a retracted position, FIG. 14, where its longitudinalaxis is perpendicular to the plane of the mounting plate, and aninserted position, FIG. 15, where its elongate axis is substantiallyparallel with the plane of the mounting plate. Rigidly attached to thechisel guide is a chisel 116 which has substantially the same shape asthe chisel 66. When the chisel guide 114 is rotated to the insertedposition the chisel substantially circumscribes the shell 10.

With this embodiment the chisel guide also acts as an impact pad.Striking the chisel guide on the outer side 118 pushes the chisel 116inwardly between the shell 10 and the pelvic bone 14, and striking thechisel guide on the inner side 120 pulls the chisel back out. Inaddition, a lever 122 located at the top of the chisel guide allows themounting plate to be rotated as the chisel is being pushed inwardly sothat the chisel does not have to continuously be pulled back, relocatedand pushed in again.

The terms and expressions which have been employed in the forgoingspecification are used therein as terms of description and not oflimitation, and there is no intention in the use of such terms andexpressions of excluding equivalence of the features shown and describedor portions thereof, it being recognized that the scope of the inventionis defined and limited only by the claims which follow.

1. A tool for removing the acetabular shell of an artificial hip, havinga semi-spheroidal outer surface and a semi-spheroidal inner surface,said tool comprising: (a) a fixture which attaches to the acetabularshell; (b) a chisel guide mounted on said fixture; (c) a chiselassociated with said chisel guide, wherein said chisel guide controlsthe movement of said chisel such that said chisel substantiallycircumscribes the outer surface of the acetabular shell when moved; (d)wherein said chisel is rigidly attached to said chisel guide; and (e)wherein the shell has an opening located at its center and said fixtureincludes an anchor which is attached to the shell through said opening.2. The tool of claim 1 wherein said anchor has a seat havingself-tapping threads and said seat is configured to be inserted intosaid opening.
 3. The tool of claim 1 wherein said opening is threadedand said anchor has a seat having threads which mate with the threads insaid opening.
 4. The tool of claim 1 wherein said fixture includes acentering plate which is circular in cross-section with a diameter whichis equal to the inner diameter of an intermediate section of the shelland has a central hole which allows said centering plate to be placedover said anchor and inserted into the shell.
 5. The tool of claim 4wherein said anchor and said central hole in said centering plate havemating threads.
 6. The tool of claim 1 wherein the shell has a radiallyextending edge and said fixture includes a cylindrical mounting platehaving a central opening which allows said mounting plate to be placedon said anchor and a peripheral base which rests on the edge of theshell.
 7. The tool of claim 6 wherein said chisel guide is rotatablymounted to said mounting plate and is moveable between a retractedposition where it is perpendicular to a top surface of said mountingplate and an inserted position where it is substantially parallel with atop surface of said mounting plate.
 8. The tool of claim 7 wherein saidchisel projects outwardly and downwardly from said chisel guide.
 9. Thetool of claim 8 wherein said chisel is arcuate along a longitudinal axisand has an inner surface with substantially the same shape as the outersurface of the shell.
 10. The tool of claim 9 wherein said inner surfaceis arcuate along a transverse axis having a radius which issubstantially equal to the radius of the outer surface of the shell. 11.The tool of claim 7 wherein said mounting plate can be rotated aboutsaid anchor.
 12. The tool of claim 11 including a striking pad locatedon said chisel guide.